1. Field of the Invention
This invention relates to a non-aqueous electrolyte secondary cell having a cathode, an anode and a non-aqueous electrolyte.
2. Description of Related Art
Among secondary cells, used as power source for portable electronic equipment, there are known a nickel-cadmium secondary cell, a nickel Metal-hydride secondary cell, a nickel zinc secondary cell and a lithium ion cell. Of these, the non-aqueous electrolyte secondary cell, employing a nickel-containing compound and a material capable of doping/undoping lithium as a cathode and an anode, respectively, or a so-called lithium ion cell, is of a high energy density and lightweight and hence is becoming a mainstream of a backup power source for e.g., a personal computer or a portable telephone set. Moreover, this non-aqueous electrolyte secondary cell is expected to be used as a secondary cell for a high performance electric car or a hybrid car.
This lithium ion cell is provided with a safety valve fitted with a mechanism for interrupting the current in case of an emergency, or with a PTC device, for the sake of safety in emergency. However, it is further desired to be improved in safety and operational reliability.
Also, a large number of lithium ion cells are assembled together as a set of unit cells for use as a secondary cell for the electric car or the hybrid car. For this reason, a higher operational reliability of a unit cell is needed for realization of the reliability as the cell seJun. 6, 2001t.
However, the conventional non-aqueous electrolyte secondary cell suffers a subject that the electrodes become expansion in charging to lead to internal shorting. For example, the electrodes tend to be expanded on initial charging to produce internal shorting to detract from operational reliability as a cell. Moreover, the electrodes are also expansion due to overcharging to occur an internal shorting, as a result of which heat is evolved within the cell to deterioration from operational safety as the cell.
It is therefore an object of the present invention to provide a non-aqueous electrolyte secondary cell progress in operational reliability and in safety.
The present invention provides a non-aqueous electrolyte secondary cell in which a spirally wound electrode unit comprised of a strip-shaped cathode and a strip-shaped anode layered together through a separator and coiled longitudinally together is sealed in a cell casing, wherein a pre-set clearance is provided between the electrode unit and the cell casing so that (B+C)/2A, where A is the inner diameter of the cell casing, B is the maximum outer diameter of the coiled electrode unit and C is the minimum outer diameter of the coiled electrode unit, is equal to or more than 0.954 and equal to or less than 0.988.
In the above-described non-aqueous electrolyte secondary cell of the present invention, in which a pre-set clearance is provided between the coiled electrode unit and the cell casing, there is no risk of the electrode unit contacting the cell casing even if the electrode unit becomes expanded due to the anode becoming expanded due to charging. Thus, with the non-aqueous electrolyte secondary cell, the inner shorting is prevented from occurring even if the coiled electrode unit becomes expansion such as due to initial charging or overcharging.
The electrode unit in the above-described non-aqueous electrolyte secondary cell has a center pin whereby minute inner shorting due to overcharging may be effectively prohibited to prevent heating to a high temperature in the cell from occurring.
That is, since the pre-set clearance is provided between the coiled electrode unit and the cell casing of the non-aqueous electrolyte secondary cell, the electrode unit may be prevented from contacting the cell casing even if the electrode is expanded due to the electrolytic solution or due to the anode becoming expanded through charging. Thus, the non-aqueous electrolyte secondary cell of the present invention is of high capacity and superior in cyclic characteristics, operational reliability and safety. In particular, the coiled electrode unit has a center pin, it is possible to prevent heat occurrence from being evolved in the cell to a high temperature due to overcharging to improve the operational safety of the non-aqueous electrolyte secondary cell having the coiled electrode unit.